In modern warfare, missiles constitute a most destructive weapon. A missile is a self-propelled guided projectile, comprising a propulsive system, an explosive head, and a guidance system housed in an equipment casing. The precision of the guidance system of a missile compared to target tracking is of utmost importance, since even a fraction of a degree of angle of error in the heading thereof can most undesirably shift the location of the area where the explosive head will detonate.
A radar is one type of guidance and target tracking system. It is a device which allows an operator to ascertain both the position and the distance of an obstacle or target, by the emission of electromagnetic waves and by the detection of the waves reflected on the surface of the obstacle. If the target being detected by the sensor is in motion relative to the radar, the echo signal will be shifted in frequency by the Doppler effect and may be used as a direct measurement of the relative target velocity. Radar principles have been applied at optical frequencies with masers (amplifiers of electromagnetic radiations with very low noise output), for the measurement of range and detection of small motions, using the Doppler-Fizeau effect.
Among known missile guidance and target tracking systems, electromagnetic sensors and optical masers (lasers) are particularly well suited. Noticeably, infra-red masers and sensors are preferred by the military because of their discretion and efficiency.
These sensors and laser emitters may be part of an electro-optical module, mounted on a missile firing vehicle such as the ADATS.TM. vehicle. Some of these sensors are highly sensitive to infra-red radiations coming from the thrust exhaust of missiles (or even aircrafts).
One major problem which relates to such modern guidance systems is the on-the-field testing of the ground based optical module bearing the emitter and sensor, and more specifically the comparison and correlation of the operating physical features of the emitter or the sensor. (These optical elements are adapted to be carried inter alia by an automotive vehicle such as the ADATS.TM. air and ground surveillance system vehicle; such vehicle also carries missile cannisters for firing missiles to be guided by the electro-optical module.) The electro-optical module is usually tested and calibrated in a relatively thorough fashion, through use of a target board, such board being located at a far distance from the module to simulate battlefield conditions. However, due to several factors such as vibrations, environmental conditions, etc . . . during transport, slight aberrations may occur and the emitter may become disaligned relative to the sensor.
Battlefield conditions are of course not favorable to such meticulous verifications of the alignment of optical module elements, and accordingly, this problem of disalignment of the operating elements of the electro-optical module between factory testing and on-the-field actual use has remained unsolved until now.
Optical devices including lasers, collimators, beam splitters and photodetectors are currently the subject of several state of the art research and development projects in the industrial world. For instance, a magneto-optical data storage device, which has recently been reported by an important scientific magazine as being developed in the Santa Clara valley in California, employs a laser to write and read data. This device writes with a laser beam, via two collimating mirrors, a beam splitter and an objective lens, by heating a spot on a rotating magneto-optical medium whose coercivity drops with increasing temperature, making it possible to magnetize the heated spot easily with a weak applied magnetic field. To read the data, the laser beam is switched to a lower intensity and polarized by the insertion of a polarizing filter between the collimating lenses and the beam splitter. Because the plane of polarization is rotated when polarized light is reflected off the magnetized medium, a second polarizing filter is positioned in register with the beam splitter opposite the objective lens, to convert the change in polarisation of the reflected beam into a change in light intensity. A photodetector assembly, positioned downstream of the second polarizer, registers changes in intensity.
Another example of such optical device is found in the U.S. Pat. No. 4,315,150 issued in 1982 to Telatemp Corp. In this prior art patent, a laser gun 10 sends a horizontal laser beam 66 toward a beam splitter 64 to strike the latter. The laser beam 66 is thereafter split orthogonally in two separate beams, to wit, a substantially horizontal beam 70 and a downwardly extending vertical beam 68. Beam 70 passes through a beam expander lens 76, which will cause this beam to be slightly refracted in a downwardly-inclined direction, and also to diverge in a conical manner so as to strike a wall 56 about a first large circular area 54. Vertical beam 68 in turn strikes an inclined mirror 34 which folds the path of that beam toward wall 56, wherein reflected horizontal beam 68 will define a second small circular area 82 within first area 54 of the expanded beam 70.
In this U.S. Pat. No. 4,315,150, the position of the beam expander 76 relative to the beam splitter 64 can be adjusted, since the former is connected to the frame of the laser gun 10 by a mounting plate 78 adjusted by screws 80. Thus, the degree of divergence of the beam 70 can be monitored. In other words, the features of the large illuminated wall area 54 can be varied relative to that of the smaller illuminated area 82, for optical alignment purposes, whereby areas 54 and 82 may correspond to the principal focus of the basic reflector structure defined by the reflectors 44 and 46 of the laser gun 10 proper, by centering zone 82 relative to zone 54. Visual discrimination would be possible by relative light intensity levels between the brighter center 82 and the peripheral area of zone 54.
Among the tests that relate to a dual sensor/emitter optical module, one may use the black body device. A black body is a standard light source, consisting of a casing having a small bore through which escapes an internal infra-red beam. It is known that an ideal blackbody has a zero reflectivity and a 100% absorptivity; i.e. would appear completely black. The interest in the blackbody lies in the character of the radiation emitted by it when heated: the total emission of radiant energy from a black body takes place at a rate expressed by the Boltzmann law, while its spectral energy distribution is described by Planck's equation. The laboratory type of black body is a hollow metal cylinder, blackened inside, and completely closed except for a narrow slit in one end. When such an enclosure is heated, the radiation escaping through the opening closely resembles the ideal blackbody radiation, while light or other radiations entering by the slit is almost completely trapped by multiple reflection from the walls, so that the opening usually appears intensely black.
One particularly efficient sensor system is defined by the forward looking infra-red system or FLIR, which allows military crew members to see at night and in poor visibility conditions. Unlike radar, the FLIR emits no energy of its own that can be detected during operations. It can locate and track vehicles and, at its maximum magnification setting, can even delineate individual tree limbs and branches.
With respect to the principle of collimators referred to hereinabove, it is well known that a collimator may be a converging lens, at one of whose focal points is placed a small source of light, usually a pinhole or narrow slit upon which light is focused from behind. Rays diverging from this focal plane emerge from the objective lens in a parallel beam. The slit or other source is viewed through the collimator without parallax, since it appears at an infinite distance.